JP4321181B2 - Method for forming an overcoat insulating film containing no chromium - Google Patents

Description

  The present invention relates to a method for forming an insulating coating on a grain-oriented electrical steel sheet, and more particularly, to a method for forming a top coating insulating coating excellent in coating characteristics and magnetic properties using a phosphate-based coating liquid not containing chromium. Is.

  The grain-oriented electrical steel sheet is generally subjected to a coating treatment on the surface of the product steel sheet in order to impart insulation, workability, rust prevention, and the like. Such a surface film is usually composed of a base film mainly composed of forsterite formed at the time of final finish annealing and a phosphate-based top coat film formed thereon.

  The surface coating is usually formed at a high temperature, but the thermal expansion coefficient of the coating is much lower than that of the steel plate. Therefore, due to the difference in thermal expansion coefficient, tensile tension is generated on the steel sheet surface after film formation. Since this tension has an effect of reducing iron loss, it is desired that the coating film formed on the surface of the electromagnetic steel sheet has a large tension imparting effect.

  In order to satisfy the above characteristics, many studies have been conducted and various coating films have been proposed. For example, in Patent Document 1, colloidal silica and one or more of aluminum phosphate, chromic anhydride and chromate are added on a glassy film formed on the surface of a steel sheet by finish annealing. A tension-applied surface coating obtained by applying and baking a coating solution is also disclosed in Patent Document 2 on a grain-oriented electrical steel sheet having a forsterite coating, colloidal silica, magnesium phosphate and chromic anhydride, chromate, There has been proposed an insulating film obtained by applying and baking a coating liquid composed of any one or more of dichromates.

  On the other hand, in recent years, products that do not contain toxic substances such as chromium and lead have been strongly desired due to waste liquid treatment, work environment and consideration of the natural environment. Surface coatings are being developed. However, the insulating coating containing no chromium has a problem of quality such as high hygroscopicity, low tension applied, and small iron loss improvement effect.

As a technique for solving the above problem, for example, in Patent Document 3, a coating liquid composed of colloidal silica, aluminum phosphate, boric acid and sulfate is applied and baked on the surface of a grain-oriented silicon steel sheet having a forsterite film. A method is disclosed. In Patent Documents 4 to 6, in a conventional tension-imparting insulating coating mainly composed of phosphate, colloidal silica, and a chromium compound, an oxide colloidal substance, a boron compound, Ca, Mn is used instead of the chromium compound. A method for adding an organic acid salt of a metal is disclosed in each.
Japanese Patent Publication No.53-28375 Japanese Patent Publication No.56-52117 Japanese Patent Publication No.57-9361 JP 2000-169972 A JP 2000-189973 A JP 2000-178760

  However, the above-described conventional film formation technology that does not contain chromium improves the iron loss improvement effect due to moisture absorption resistance and tension application to a level close to that of the conventional chromium-containing film, but the effect varies greatly. In some cases, iron loss and moisture absorption resistance may deteriorate so that it cannot be commercialized, which is insufficient for practical use.

  An object of the present invention is to provide an advantageous method of forming a top insulating film having excellent film characteristics and magnetic property improvement effects equivalent to or better than those of conventional chromium-containing films, using a phosphate-based coating solution containing no chromium. It is to propose.

  The inventors consider that the above-mentioned problems of the coating film not containing chromium, that is, the problem of large variation in moisture absorption resistance and iron loss improvement effect, cannot achieve desired characteristics due to some disturbance factor. , Earnestly researched to investigate the cause. As a result, it was determined that the atmospheric conditions when applying and baking the overcoat insulating coating on the surface of the grain-oriented electrical steel sheet after finish annealing were the main factors causing the above-mentioned variation, and the present invention was completed. .

That is, the present invention provides a method of applying a phosphate-based coating liquid not containing chromium on the surface of a grain-oriented electrical steel sheet after finish annealing and baking it by flattening annealing to form a flat insulating film. Chromium is characterized in that it is carried out in an atmosphere in which the average temperature increase rate at 200 to 700 ° C. is 10 to 60 ° C./sec and the oxygen potential P _H2O / P _H2 in the soaking temperature range is 0.2 or less. This is a method for forming an overcoat insulating film that does not contain.

  The grain-oriented electrical steel sheet after finish annealing in the present invention works effectively even if it does not have a forsterite film on the surface.

  ADVANTAGE OF THE INVENTION According to this invention, even if it uses the phosphate type coating liquid which does not contain chromium, the grain-oriented electrical steel sheet excellent in both the film characteristic and the magnetic characteristic can be obtained stably. In addition, since the insulating coating can be dechromed, it greatly contributes to simplification of waste liquid treatment, improvement of the working environment, protection of the natural environment, and the like.

First, an experiment that triggered the development of the present invention will be described.
C: 0.045mass%, Si: 3.25mass%, Mn: 0.07mass%, Se: 0.02mass%, the remainder of the silicon steel slab consisting essentially of Fe is heated at 1380 ° C for 30 minutes and then hot Rolled into a hot-rolled steel sheet with a thickness of 2.2 mm. This hot-rolled steel sheet was subjected to hot-rolled sheet annealing at 950 ° C. for 1 minute, and then cold-rolled twice at 1000 ° C. for 1 minute. A cold-rolled steel sheet having a final thickness of 0.23 mm was obtained. Thereafter, this cold-rolled steel sheet was decarburized and annealed at 850 ° C. for 2 minutes in a mixed atmosphere of H ₂ and N ₂ with an oxygen potential of P _H20 / P _H2 : 0.55, and then 100 parts by weight of magnesium oxide, titanium oxide An annealing separator consisting of 2 parts by weight and 1 part by weight of strontium sulfate is coated and dried on both sides of the steel sheet at a rate of 12 g / m ² , followed by secondary recrystallization annealing and 1200 ° C for 10 hours in a dry H ₂ atmosphere. Finished annealing consisting of the above-mentioned purification annealing was performed, and the unreacted separating agent was removed by phosphoric acid pickling to obtain a grain-oriented electrical steel sheet having a glassy forsterite film. The grain-oriented electrical steel sheet obtained as described above is sheared to a size of 300 mm x 100 mm, and the magnetic properties (iron loss) are measured using an SST tester (Single Sheet Tester). It was.

Thereafter, the grain-oriented electrical steel sheet after the phosphoric acid pickling is composed of 40 parts by weight of colloidal silica, 50 parts by weight of aluminum phosphate, 5 parts by weight of boric acid, and the same component as the coating solution disclosed in Patent Document 3. After applying a phosphate coating liquid consisting of 10 parts by weight of magnesium sulfate to a dry weight of 10 g / m ^{2 on} both sides, 90% N ₂ for both baking of the coating liquid and flattening annealing Annealing was performed at 800 ° C. for 2 minutes in a + 10% H ₂ atmosphere. During the annealing, the oxygen potential P _H20 / P _H2 was changed in the range of 0.05 to 0.5 by changing the dew point of the atmosphere in the soaking temperature range. For comparison, a coating solution belonging to the prior art consisting of 40 parts by weight of colloidal silica, 50 parts by weight of aluminum phosphate, and 10 parts by weight of chromic anhydride was also applied and baked by the same method, and flattening annealing was performed. The steel sheet thus obtained was again measured for magnetic properties with an SST tester and subjected to a P dissolution test. In the P dissolution test, three 50 mm × 50 mm test pieces were immersed and boiled in distilled water at 100 ° C. for 5 minutes to elute P from the surface of the film, and quantitatively analyze the eluted P. The elution amount indicates the ease of dissolution of the film in water and is an effective index for evaluating the moisture absorption resistance of the film.

Regarding the results of the above test, FIG. 1 shows the relationship between the oxygen potential P _H20 / P _H2 and the iron loss W _17/50, and FIG. 2 shows the relationship between the oxygen potential P _H20 / P _H2 and the P elution amount. From these figures, in the steel plate coated with the conventional chromium-containing coating solution, even if the oxygen potential of the atmosphere changes, the magnetic properties and P elution amount do not change greatly, while maintaining a good value, In the steel plate coated with phosphate-based coating solution that does not contain chromium, the iron loss and P elution amount are as good as those of the chromium-containing coating solution in the region where the oxygen potential P _H20 / P _H2 is low. However, it was found that when the oxygen potential P _H20 / P _H2 exceeds 0.2, both the iron loss and the elution amount of P increase rapidly, and the magnetic properties and moisture absorption resistance deteriorate rapidly.

The inventors consider the mechanism of the change in film properties due to the oxygen potential in the annealing atmosphere as follows.
First, in the conventional chromium-containing coating solution, phosphate and colloidal silica react, and the remaining free phosphoric acid is trapped by chromium. As a result, elution of P is suppressed and moisture absorption resistance is improved. Is done. Furthermore, when the oxygen potential P _H20 / P _H2 is high, even if cracks occur during drying, the cracks are easily repaired by the chromium repair function, so the starting point of P elution is reduced. To do. In addition, since a strong film is formed by repairing cracks, it is considered that the tension effect on the steel sheet is increased and the magnetic properties (iron loss) are improved.

In contrast, the phosphate-based coating liquid that does not contain chromium used in the above experiment has a function of trapping free phosphoric acid in metal sulfate and boric acid instead of chromium. When the oxygen potential P _H20 / P _H2 of the atmosphere during annealing is high, the oxygen content in the atmosphere damages the coating, so that cracks are likely to occur, and the effect of trapping phosphoric acid is insufficient. In addition, the phosphate-based coating solution that does not contain chromium does not have a self-healing function of chromium, so cracks may occur in the baked coating film or the underlying forsterite film, or peeling may occur. . In addition, when analyzing the periphery of cracks in phosphate coating films that do not contain chromium, in many cases phosphorus is concentrated and the amount of Si is small, so free phosphoric acid is strongly concentrated in this area. It is expected that As a result, if there is a crack or a peeled portion, free phosphoric acid is eluted from this portion, which is considered to deteriorate the moisture absorption resistance. Moreover, since the presence of cracks as described above also weakens the strength of the coating itself, it is considered that the effect of imparting tension is reduced and the effect of improving iron loss is lost.
On the other hand, when the oxygen potential P _H20 / P _H2 is low, since the atmospheric oxygen content is low, the generation of cracks is suppressed, thereby reducing the starting point from which P is eluted, improving the moisture absorption resistance, and the coating film. Since the strength itself can be secured, it is considered that the original tension effect is exhibited and the iron loss is effectively reduced.

Next, the formation method of the top coat insulating film according to the present invention will be described.
In the present invention, the material of the grain-oriented electrical steel sheet for forming the top coat is not particularly limited as long as it is a conventionally known material for grain-oriented electrical steel sheets containing Si. The production method of the grain-oriented electrical steel sheet is not particularly limited, and the above-described conventionally known steel slab is heated by a conventionally known method, hot-rolled, and then subjected to hot-rolled sheet annealing as necessary, and thereafter Rolled to the final plate thickness by cold rolling multiple times with one or more intermediate annealings, and after applying primary recrystallization annealing that also serves as decarburization, apply an annealing separator mainly composed of magnesia, and then secondary It is preferable to perform finish annealing that combines recrystallization annealing and purification annealing.

  In the case of a normal grain-oriented electrical steel sheet, a vitreous forsterite film is formed on the surface of the steel sheet after finish annealing. However, after decarburization annealing, alumina is used as an annealing separator to be applied to the steel sheet surface, or a forsterite film is hardly formed on the surface by using a separator containing magnesia added with chloride. Techniques for improving punchability and magnetic properties have also been developed. The present invention can also be effectively applied to a steel sheet that does not form such a coating. The reason for this is that since annealing is performed under conditions with a low oxygen potential, there is no quality deterioration due to oxidation of the steel sheet, and cracks are not generated in the coating, so there is little deterioration in corrosion resistance even without a forsterite film.

  Then, the phosphate coating liquid which does not contain chromium is apply | coated to the surface of the said directionally annealed grain-oriented electrical steel sheet. Examples of the coating liquid include conventionally known coating liquids such as a coating liquid composed of colloidal silica and aluminum phosphate, boric acid and sulfate disclosed in Patent Document 3, and phosphoric acid disclosed in Patent Documents 4 to 6. Phosphate-based phosphate-free coating solution containing boric acid compound, oxide colloid or metal organic acid salt, etc. instead of chromic acid, in coating solution containing salt, colloidal silica and chromium compound as main components Any coating solution can be suitably used. Furthermore, in the present invention, the sticking resistance may be improved by adding inorganic mineral particles such as silica and alumina to the coating solution.

Note that the basis weight of the phosphate-based coating liquid not containing chromium is preferably in the range of 4 to 15 g / m ^{2 on} both sides. This is because if it is less than 4 g / m ² , the interlayer resistance decreases, and if it exceeds 15 g / m ² , the space factor decreases, which is not preferable.

After the coating liquid is applied and dried, annealing that combines baking and planarization annealing (hereinafter simply referred to as “planarization annealing”) is performed. It is particularly important in the present invention that the atmosphere in the soaking temperature region of this flattening annealing is 0.2 or less in terms of oxygen potential P _H20 / P _H2 . When P _H20 / P _H2 exceeds 0.2, as described above, free phosphoric acid traps become insufficient, and cracks are likely to occur in the coating, resulting in deterioration of coating properties and magnetic properties. .

Although the other conditions in planarization annealing are not specifically limited, The average temperature increase rate of 200-700 degreeC needs to be 10-60 degreeC / sec. This is because if the rate of temperature rise is lower than 10 ° C./sec, the coating tends to swell due to the generation of gas such as water vapor, and conversely if it exceeds 60 ° C./sec, cracks are likely to occur in the coating. Moreover, it is desirable that the annealing condition is a temperature of 700 to 950 ° C. and a time of about 2 to 120 seconds. If the temperature is too low or the time is too short, flattening is insufficient and shape defects tend to occur.On the other hand, if the temperature is too high or the time is too long, the effect of flattening annealing is too strong and creep occurs. This is because it deforms and causes deterioration of the magnetic characteristics.

Magnetic domain refinement process in which grooves with a depth of 20μm are formed at a pitch of 4mm on a silicon-containing cold-rolled steel sheet (Si: 3.2mass%) rolled to a thickness of 0.23mm in a direction inclined 10 ° to the direction perpendicular to the rolling direction After decarburization, decarburization annealing was performed at 850 ° C. for 2 minutes in a mixed atmosphere of H ₂ and N ₂ having an oxygen potential P _H20 / P _H2 of 0.55, and then 3 parts by weight of nickel chloride per 100 parts by weight of magnesia. After applying and drying an annealing separator mainly composed of a powder added with a powder, finish annealing is performed by a known method, and then the unreacted separating agent is removed by phosphoric acid pickling so that there is no forsterite film. A grain-oriented electrical steel sheet was obtained. The magnetic flux density B ₈ of the steel sheet after the final annealing was 1.89T. Then, on the surface of the steel sheet after the phosphoric acid pickling treatment, the component composition is a dry solid ratio, colloidal silica: 50 parts by weight, magnesium phosphate: 40 parts by weight, manganese sulfate: 9.5 parts by weight, fine powder silica particles : Phosphate-based coating solution consisting of 0.5 parts by weight was applied at 10 g / m ² on both sides, heated to 200-700 ° C at a heating rate of 10 ° C / sec, soaking temperature: 850 ° C, soaking Heating time: A flattening annealing of 30 seconds was performed. In the planarization annealing, the oxygen potential P _H20 / P _H2 of the atmosphere in the soaking temperature range was changed in the range of 0.05 to 0.45. About the steel plate obtained as mentioned above, it investigated in the following way about powdering property, heat resistance, a magnetostriction characteristic, adhesiveness, a space factor, an external appearance, rust prevention property, and P elution amount. Further, the iron loss was measured after finish annealing and after the formation of the top coat.

Iron loss: Iron loss W _17/50 was measured using an SST tester.
・ Powdering property: Using SEM, observe the surface of the film at a magnification of 500 times, ○ if there is no swelling or cracking on the surface, △ if there is a slight swelling or cracking, × if there is severe swelling or cracking × It was evaluated.
・ Heat resistance (sticking resistance): Ten test pieces of 50 mm x 50 mm in size were closely stacked and subjected to annealing at 800 ° C for 2 hours in a dry nitrogen atmosphere with a 20 MPa compression load applied. Later, when a weight of 500 g was dropped on the stacked specimen, the fall height of the weight when all the stacked specimens were separated was measured, and the height was less than 20 cm ○, 20 cm or more and less than 40 cm Was evaluated as Δ, and 60 cm or more was evaluated as ×.
Magnetostriction: Compressive stress (MPa) at which λ _PP is 4 × 10 ⁻⁴ was measured and evaluated.
-Adhesiveness: A test piece was wound around a round bar having various diameters, and the minimum bending diameter (diameter) at which peeling did not occur was evaluated.
-Space factor: Measured according to the space factor test specified in JIS C 2550.
Appearance: The beauty of the coating surface was evaluated by visual observation.
・ Rust prevention: After holding in an air atmosphere at a temperature of 50 ° C and a dew point of 50 ° C for 50 hours, the surface is visually observed, ○ if there is no rust, △ if there is some rust, rust is severe Things were rated as x.
P dissolution test: Three test pieces of 50 mm × 50 mm were immersed and boiled in 100 ° C. distilled water for 5 minutes, and P eluted from the coating surface was quantitatively analyzed and evaluated.

The results of the above test are shown in Table 1. The coating liquid for the top coat insulating film of this example is the phosphate-based coating liquid disclosed in Patent Document 3 with no addition of boric acid. In addition, by annealing under the conditions of the present invention, that is, the oxygen potential P _H20 / P _H2 of the atmosphere is 0.2 or less, a directional electromagnetic wave excellent in film properties (moisture absorption resistance, adhesion, etc.) and magnetic properties is obtained. A steel plate can be obtained.

After subjecting a silicon-containing grain-oriented electrical steel sheet with a forsterite coating (Si: 3.3 mass%) with a magnetic flux density B ₈ of 1.89 T after final finish annealing to a phosphoric acid pickling treatment, the component composition is in the ratio of dry solid to solid , Colloidal silica: 50 parts by weight, various first phosphate compounds: 40 parts by weight, various inorganic compounds: 9.5 parts by weight, fine powder silica particles: 0.5 parts by weight After applying 10 g / m ² on both sides of the acid salt compound and inorganic compound), heat up to 200-700 ° C at a heating rate of 20 ° C / sec, and set the oxygen potential P _H20 / P _H2 Flattening annealing at 850 ° C. for 30 seconds was performed in an atmosphere controlled to 0.06 or 0.32.

About the steel plate obtained as mentioned above, various characteristics were investigated like Example 1, and the result was combined with Table 2 and shown. From this result, the oxygen potential P _H20 / P _H2 of the atmosphere at the time of flattening annealing is within an appropriate range even when using the phosphate-based coating liquid not containing chromium described in Patent Documents 4 to 6 and the like. It turns out that the grain-oriented electrical steel sheet which has the outstanding magnetic characteristic and film characteristic is obtained by making it reduce.

The technology of the present invention can be used not only for grain-oriented electrical steel sheets but also for non-oriented electrical steel sheets.

Oxygen potential P _H20 / P _H2 atmosphere in the flattening annealing is a diagram showing the effect on iron loss. Oxygen potential P _H20 / P _H2 atmosphere in the flattening annealing is a diagram showing the effect on P elution amount of the film.

Claims (1)

In the method of applying a phosphate-based coating liquid not containing chromium to the surface of a grain- oriented electrical steel sheet after finish annealing and baking it by flattening annealing to form a top insulating film, the flattening annealing of 200 to 700 Chromium-free overcoat insulating film, characterized in that the average temperature rising rate at 10 ° C. is 10 to 60 ° C./sec and the oxygen potential P _H2O / P _H2 in the soaking temperature region is 0.2 or less. Forming method.

Images